Modern wireless communication devices (WCD) are required to operate in a wide frequency range, multiple frequency bands and multiple operational modes. WCDs typically include a number of Radio Frequency (RF) receivers, RF transmitters, a baseband gain amplifier with filtering (BB) and baseband processors (BP). Each RF receiver is configured for receiving RF signals within a specified band. An RF receiver typically includes a radio frequency (RF) front end (RF FE) to down-convert the RF signal through a mixer either to Intermediate Frequency (IF) or to Zero-Intermediate-frequency (ZIF). The terms “RF receiver”, “receiver” and “RF FE” are used interchangeably in this disclosure. However, one skilled in the art may appreciate that a receiver may also comprise other components, such as low pass filters or amplifiers.
An RF FE includes at least a low noise amplifier (LNA) and a mixer. RF receivers require different levels of amplification for each band and operational mode. Traditional amplification solutions employ multiple circuit blocks or modules, each with a different amplification scheme, to accommodate for the various standards. Furthermore, the use of RF output stage switches is necessary to select between the amplification schemes for each band or mode. Multi-band, multi-mode RF receivers using multiple LNAs and mixers require a large semiconductor die area and long inter-connect routings. It is a challenge to maintain low signal loss, low parasitic resistance and capacitance for such routings, since any parasitic resistance and capacitance will either introduce noise or limit the tuning range of the LNAs.
To reduce the number of LNAs, tuned multi-band, multi-mode RF FEs for RF receivers have been proposed. These proposals suggest using one LNA with a tunable LC load. The LNA may operate in current mode when a low impedance load is added to its output. FIG. 1 shows an LNA for use in a multi-band multi-mode WCD. LNA 110 includes transconductor 112, for converting the RF input voltage signal to current, inductor 114, inductor parasitic resistor 116 and capacitor 121. Inductor 114 and capacitor 121 are tunable. Thus, LNA 100 may be tuned to the desired frequency. LNA 110 is suitable for a multi-band multi-mode WCD. With this configuration, a single LNA operates over multiple frequency bands without the use of multiple LNAs. Consequently, it will not require any switches at the output of the RF FE to switch between the multiple LNAs. Tunable capacitor 121, typically, comprises a capacitor bank programmable through a plurality of switches. Typically, one switch per capacitor of the capacitor bank is used for every tunable configuration. When tuning the RF FE of a WCD that includes an LNA such as LNA 110, great care must be taken as the current transfer gain of the RF FE (“RF FE gain”) and overall receiver noise figure (“receiver NF”) vary as the capacitance of capacitor 121 varies.
For a given frequency and for a fixed inductance L of inductor 114, optimizing RF FE gain or receiver NF would require varying the capacitance of capacitor 121. This in turn would result in departing from the central frequency, particularly in wide-band receivers. In multi-band, multi-mode systems, it is a design challenge to optimize either for RF FE gain or for receiver NF without affecting the RF FE central frequency.
It would be advantageous to provide for a capacitor bank architecture that allows controllable optimization of RF FE gain and receiver NF in a multi-band, multi mode WCD receiver.